In general (not answering this specific case), a fund wants more money under management, because they get percentages of more money that way (they have more leverage). You might be assuming Goldman has infinite money - they don't. Here's how to think of this:
Say I'm an investing wizard - I can generate 20% returns on any money invested.
Scenario 1: I have 100k capital, which means after one year, I'll have 120k. I made 20%, but in practice that's 20k.
Scenario 2: Now say I sell to you my wizarding ability by giving you 10% and keeping 10%. I'm giving you half the upside. However, you are a rich bank that invests 100m. I turn it after one year into 120m, give you half of that and keep half, so you've gotten 10m and I've gotten 10m.
Now percentage-wise, obviously I'm ahead in scenario 1, but in real terms, obviously I prefer scenario 2 - I've gotten 10m dollars instead of 20k dollars.
Btw in the real world, if I have to invest more money, my prowess goes down. So with 100k I can do 20%, with 1m I can do say 19%, etc. So what you'd expect to have happen is that more and more people will invest, because it's worth it to them, until eventually my abilities go down to just generic S&P level. Up to then, it's worth it for every extra investor to invest, because even if I'm only beating the market by 1%, that's a lot, but eventually I'm managing enough money such that I'm just rivaling the S&P. So from the outside, it looks like I'm not really doing anything special, but it was totally worth it for every investor until now.
You are guaranteed alpha at exactly 1%. Goldman makes fees and infinity ROI for taking risk. Win-Win.